U.S. Pat. No. 5,914,218 (Smith et al.) describes photolithographically patterned spring structures for use in the production of low cost probe cards, to provide electrical connections between integrated circuits, or to form coils that replace surface-mount inductors. A typical spring structure includes a spring finger having an anchor portion secured to a substrate, and a free portion initially formed on a pad of release material. The spring finger is etched from a thin spring material layer (film) that is fabricated such that its lower portions have a higher internal compressive stress than its upper portions, thereby producing an internal stress gradient that causes the spring finger to bend away from the substrate when the release material is etched. The internal stress gradient is produced in the thin spring material film either by layering different materials having the desired stress characteristics, or using a single material by altering the fabrication parameters.
A problem with high-volume production of integrated circuits incorporating photolithographically patterned spring structures is that the released “free” portions of some spring structures fabricated according to conventional methods undergo helical twisting, thereby skewing (displacing) the spring structure tips from their intended position. A spring structure is typically designed to curl or bend perpendicular to the underlying substrate (i.e., in a plane passing through the spring structure's longitudinal axis) upon release such that the tip is located in a predefined position above the substrate. The tip's position is typically matched to a receiving structure (e.g., a contact pad) formed on an integrated circuit to which the spring structure is electrically connected. Helical twisting causes the spring structure to bend such that the tip is positioned away from the predefined position, thereby preventing optimal connection between the spring structure and the receiving structure. To make matters worse, the amount of skew tends to vary according to orientation of the spring structure, and spatially over the wafer upon which the spring structures are produced in high volume. That is, in one region of a wafer, spring structures oriented in a particular direction may experience a relatively small amount of twisting, while spring structures in that region oriented in another direction experience pronounced twisting. Also, similarly oriented spring structures that are located in different regions may experience different amounts of twisting. The amount of skew can even be zero in certain locations and orientations.
The amount of skew that can be tolerated in a spring structure depends critically on the application in which the spring structure is used. For the manufacture of self-assembling out-of-plane inductors, for example, the specification for the skew is the lesser of ±1% of the spring diameter or ±5 microns. For other applications, such as packaging, the specification may be a little less stringent, and will depend on the size and spacing of the pads that the springs are designed to contact.
As suggested above, one solution to problems facing high-volume production of integrated circuits incorporating photolithographically patterned spring structures is to design systems that take into account the expected range of spring structure skew (which would be determined experimentally before high-volume production is initiated). However, this solution generates inefficiencies (e.g., wider spring structure spacing and larger contact pads) that increase production costs. Another possible solution would be to identify the locations and orientations on the wafers at which zero skew occurs in a given fabrication process, and then only fabricate spring structures in these zero skew locations. However, this solution would limit the wafer area utilized to fabricate spring structures, thereby making high-volume production expensive and complicated.
What is needed is a method for fabricating spring structures that minimizes or eliminates helical twisting, thereby facilitating high-volume production.